(1) Field of the Invention
The present invention relates to a semiconductor laser element and a method for producing the semiconductor laser element. More particularly, the present invention relates to a semiconductor laser element suitable for a light source of an optical communication and a light source of an optical disc, and relates to a method for producing the semiconductor laser element.
(2) Description of Related Art
Typically, conventional semiconductor laser element is composed of a p-type electrode, an n-type electrode, a p-type light waveguide, an n-type light waveguide, and an active layer.
The p-type electrode, n-type electrode, p-type light waveguide, n-type light waveguide are disposed in the vicinity of the active layer, and are made of different materials from the active layer. With such a construction, light generated in the active layer is confined in the semiconductor laser element, a reduced emission is generated in the active layer, and laser light is generated. There is also known a semiconductor laser element that amplifies light with a certain wavelength, using Fabry-Perot resonator in which a cleavage plane is used as a resonator.
It is desired however that the conventional semiconductor laser elements have a low-threshold-current characteristic to be under improved control when they are used as light sources for light communications or optical discs.
It is therefore an object of the present invention to provide a semiconductor laser element having an excellent light confinement effect and a low-threshold-current characteristic, and a method for producing the semiconductor laser element.
Recently, an artificially created dielectric constant three-dimensional periodic structure has received attention. This structure controls the movement of electromagnetic waves so as to move like electrons in crystals. This artificially created structure is called three-dimensional photonic crystal. The electromagnetic band that is caused by this structure and is equivalent to a band of light is called a photonic band.
The reason why the photonic band structure is receiving attention is that the structure may create a laser light whose output or wavelength does not change much with the change of temperature since it has a very low threshold value (theoretically it has no threshold value) and can perfectly control photons in space, which has been impossible for conventional techniques. Also, the photonic band structure is superior in electricity-light energy exchange efficiency since it controls emitted light in all directions in space. In other words, the photonic band structure may provide a low-power-consumption laser. It is thought that especially, a three-dimensional photonic band structure having the dielectric constant three-dimensional periodic structure provides the above effect at the maximum.
FIG. 14 shows an example of the three-dimensional photonic crystal structure (disclosed in the Japanese Laid-Open Patent Application No. 10-335758).
The three-dimensional photonic crystal structure shown in FIG. 14 is made by periodically stacking layers of two or more kinds of materials which each periodically have pits and projections. For example, Si and SiO2 are used as the materials for the layers 1401 and 1402, and are referred to as A1401 and B1402, respectively. With such a three-dimensional structure, the propagation of light having a certain wavelength is cut and confined in the crystal structure due to the difference between the refractive indexes of light of the materials A1401 and B1402. However, it is substantially impossible to achieve a semiconductor laser element by forming an active layer inside the above three-dimensional photonic crystal structure. Also, even if the active layer is formed inside the structure, it is difficult to form a waveguide for obtaining light.
In consideration of the above problems, the inventors of the present invention conceived a semiconductor laser element that uses the excellent light confinement effect of the three-dimensional photonic crystal structure by forming inside the three-dimensional photonic crystal structure an active unit that generates laser beams after receiving power, the three-dimensional photonic crystal structure being made of a stack of a plurality of refractive index changing layers in which a refractive index of light periodically changes in a certain direction.
The above object is fulfilled by a semiconductor laser element comprising: a three-dimensional photonic crystal structure which has a light confining effect and includes alternating first and second refractive index changing layers, wherein refractive index of light periodically changes in a first direction in each first refractive index changing layer and periodically changes in a second direction in each second refractive index changing layer; and an active unit which is disposed in a portion having a predetermined refractive index inside the three-dimensional photonic crystal structure, and generates a laser beam in response to reception of electric power.
With the above construction, it is possible to achieve a semiconductor laser element that has an excellent effect of confining the light generated in the active layer and a low-threshold-current characteristic since it uses the light confinement effect of the three-dimensional photonic crystal structure. Also, the three-dimensional photonic crystal structure of the present invention uses, instead of the conventional honeycomb-layer stack structure, first refractive index changing layers and second refractive index changing layers which are alternately stacked, where the refractive index of light periodically changes in a first direction in the first refractive index changing layers, and changes in a second direction in the second refractive index changing layers. This enables the active layer to be formed during the layer stacking process, achieving a semiconductor laser element using the three-dimensional photonic crystal structure.
In the above semiconductor laser element, the active unit may be disposed substantially at a center of the three-dimensional photonic crystal structure.
With the above construction, the active unit is disposed at the center of the three-dimensional photonic crystal structure. This enhances the light confinement effect and decreases the threshold of the electric current.
The above semiconductor laser element may further comprise a light waveguide which extends horizontally from an end of the active unit to at least a vicinity of an end of the three-dimensional photonic crystal structure.
In the above semiconductor laser element, each first refractive index changing layer may be composed of a plurality of optically refractive stripes arranged parallel to each other with a predetermined pitch so that refractive index of light periodically changes in the first direction, each second refractive index changing layer is composed of a plurality of optically refractive stripes arranged parallel to each other with substantially the same pitch as the predetermined pitch so that refractive index of light periodically changes in the second direction, phase of period of the plurality of optically refractive stripes constituting one refractive index changing layer is different from a phase of period of the plurality of optically refractive stripes constituting adjacent refractive index changing layer, and a laser emitting stripe that includes the active unit is disposed in place of one optically refractive stripe.
In the above semiconductor laser element, the active unit may be disposed substantially at a center of the laser emitting stripe in the direction of length, and two portions ranging from both ends of the active unit to both ends of the laser emitting stripe are a p-type light waveguide and an n-type light waveguide, respectively, the p-type and n-type light waveguides each doubling as a carrier conduction path.
The above semiconductor laser element may further comprise: a p-type carrier conduction path formed to overlap the p-type light waveguide; an n-type carrier conduction path formed to overlap the n-type light waveguide; a p-type contact layer formed to overlap the p-type light waveguide; a p-type electrode formed to overlap the p-type light waveguide; an n-type contact layer formed to overlap the n-type light waveguide; and an n-type electrode formed to overlap the n-type light waveguide.
The above semiconductor laser element may further comprise a high-reflection layer covering a side of the laser emitting stripe at an end in the direction of length.
In the above semiconductor laser element, the high-reflection layer may be composed of a plurality or dielectric layers.
In the above semiconductor laser element, the high-reflection layer may include at least a SiO2 layer and an amorphous Si layer, in the order from an end of the laser emitting stripe.
In the above semiconductor laser element, each optically refractive stripe may be made of a material selected from the group consisting of InP, GaAs, AlGaInP, MgZnSSe, and AlGaN.
In the above semiconductor laser element, each first refractive index changing layer may be composed of (a) a plurality of optically refractive stripes arranged parallel to each other with a predetermined pitch and (b) a plurality of dielectric stripes displacing space between the plurality of optically refractive stripe so that refractive index of light periodically changes in the first direction, each second refractive index changing layer is composed of (c) a plurality of optically refractive stripes arranged parallel to each other with substantially the same pitch as the predetermined pitch and (d) a plurality of dielectric stripes displacing space between the plurality of optically refractive stripes in (c) so that refractive index of light periodically changes in the second direction, phase of period of the plurality of optically refractive stripes constituting one refractive index changing layer is different from phase of period of the plurality of optically refractive stripes constituting adjacent refractive index changing layer, and one optically refractive stripe has been replaced with a laser emitting stripe that includes the active unit.
With the above construction, the semiconductor laser element is prevented from being oxidized by air. This improves the reliability of the element.
The above object is also fulfilled by a method of producing the above semiconductor laser element, characterized by an assembling step for systematically arranging and assembling a plurality of optically refractive stripes and a laser emitting stripe including an active unit that generates a laser beam in response to reception of electric power.
With the above construction, it impossible to form the active unit during the process for stacking each component constituting the three-dimensional photonic crystal structure, achieving a semiconductor laser element using the three-dimensional photonic crystal structure. This achieves a semiconductor laser element that has an excellent effect of confining the light generated in the active layer and a low-threshold-current characteristic since it uses the light confinement effect of the three-dimensional photonic crystal structure.
In the above semiconductor laser element production method, the assembling step may include: a first large step for forming a refractive index changing layer by arranging optically refractive stripes in parallel to each other so that refractive index of light periodically changes in a predetermined direction; a second large step for forming a laser emitting layer in which refractive index of light periodically changes in a predetermined direction, by arranging the laser emitting stripe and optically refractive stripes in parallel to each other; and a third large step for stacking the refractive index changing layer and the laser emitting layer.
With the above construction, it is possible to form the active unit during the process for stacking each component constituting the three-dimensional photonic crystal structure, and also possible to achieve a semiconductor laser element using the three-dimensional photonic crystal structure without difficulty since it uses, instead of the conventional honeycomb-layer stack structure, first refractive index changing layers and second refractive index changing layers which are alternately stacked, where the refractive index of light periodically changes in a first direction in the first refractive index changing layers, and changes in a second direction in the second refractive index changing layers.
In the above semiconductor laser element production method, in the first large step, the refractive index changing layer may be formed by arranging optically refractive stripes in parallel to each other with a predetermined pitch so that refractive index of light periodically changes, the optically refractive stripes having a different refractive index of light from air layers between the optically refractive stripes, and in the second large step, the laser emitting layer is formed by arranging optically refractive stripes in parallel to each other with the same pitch as the refractive index changing layer, disposing the laser emitting stripe at a center of the optically refractive stripes instead of an optically refractive stripe.
In the above semiconductor laser element production method, the second large step may include: a first step for forming an etching stop layer on a surface of a substrate; a second step for forming on a surface of the etching stop layer an active unit area whose shape is similar to that of the active unit; a third step for forming on the surface of the etching stop layer a p-type light waveguide area and an n-type light waveguide area in the vicinity of the active unit area, shapes of the p-type light waveguide area and the n-type light waveguide area being similar to those of a p-type light waveguide and an n-type light waveguide, respectively; a fourth step for forming on the surface of the etching stop layer an optically refractive material layer to enclose the active unit area, the p-type light waveguide area, and the n-type light waveguide area; and a fifth step for forming the laser emitting layer by shaping the optically refractive material layer into a plurality of stripes with the same period as each refractive index changing layer, using a patterning method, wherein the third large step includes a removing step for removing the etching stop layer and the substrate after the retractive index changing layer and the laser emitting layer are stacked.
In the above semiconductor laser element production method, the second step may include: a first small step for forming on the surface of the etching stop layer a layer of a material of the active unit; and a second small step for forming the active unit area by partially removing the layer using a patterning method.
In the above semiconductor laser element production method, the second step may include: a first small step for forming on the surface of the etching stop layer a layer of an insulating material; a second small step for forming a pit in the insulating material layer; a third small step for forming the active unit area in the pit by an organometallic vapor deposition method; and a fourth small step for removing the insulating material layer after the active unit area is formed.
In the above semiconductor laser element production method, the active unit may be a quantum well type, and the second step may include: a first small step for forming an etching stop layer on a surface of a substrate; a second small step for forming on a surface of the etching stop layer a stripe of a material of the active unit having a size enough to include the active unit, a p-type light waveguide, and an n-type light waveguide; a third small step for introducing impurities into the active unit material stripe except for a center portion thereof so that two impurities-introduced portions around the center portion become a p-type light waveguide area and an n-type light waveguide area, and so that the center portion becomes an active unit area; a fourth small step for forming on the surface of the etching stop layer an optically refractive material layer to enclose the active unit area, the p-type light waveguide area, and the n-type light waveguide area; and a fifth step for forming the laser emitting layer by shaping the optically refractive material layer into a plurality of stripes with the same period as the refractive index changing layer, using a patterning method, wherein the third large step includes a removing step for removing the etching stop layer after the refractive index changing layer and the laser emitting layer are stacked.
In the above semiconductor laser element production method, the second step may include: a first small step for forming on the surface of the etching stop layer a layer of a material of the active unit; and a second small step for forming the active unit material stripe by partially removing the layer using a patterning method.
In the above semiconductor laser element production method, the second step may include; a first small step for forming on the surface of the etching stop layer a layer of an insulating material; a second small step for forming a channel in the insulating material layer; a third small step for forming the active unit area in the channel by an organometallic vapor deposition method; and a fourth small step for removing the insulating material layer after the active unit area is formed.
The above semiconductor laser element production method may further comprise a conduction path forming step for forming the p-type carrier conduction path and the n-type carrier conduction path to overlap the p-type light waveguide and the n-type light waveguide, respectively, after a three-dimensional photonic crystal structure is formed by stacking the refractive index changing layer and the laser emitting layer.
In the above semiconductor laser element production method, the assembling step may include: a first large step for forming a refractive index changing layer by arranging (a) a plurality of optically refractive stripes in parallel to each other and (b) a plurality of dielectric stripes displacing space between the plurality of optically refractive stripes so that refractive index of light periodically changes in a predetermined direction; a second large step for forming a laser emitting layer in which refractive index of light periodically changes in a predetermined direction, by arranging the laser emitting stripe and optically refractive stripes in parallel to each other and arranging a plurality of dielectric stripes to displace space between the laser emitting stripe and the optically refractive stripes so that refractive index of light periodically changes in a predetermined direction; and a third large step for stacking the refractive index changing layer and the laser emitting layer.